The disclosure relates generally to an electrostatographic printer or copier, and more particularly concerns a device for removing or preventing transfer streaks on a printout.
In an electrophotographic application such as xerography, a charge retentive surface (i.e., photoconductor, photoreceptor or imaging surface) is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided, electrostatically attractable powder referred to as “toner”. Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to transfer, excess toner left on the charge retentive surface is cleaned from the surface. This process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be image-wise discharged in a variety of ways. Ion projection devices where a charge is image-wise deposited on a charge retentive substrate operate similarly.
One type of charge retentive surface typically utilized in the electrostatographic reproduction device is a photoreceptor belt having a base of flexible material. The photoreceptor belt is entrained about a plurality of support rollers and/or stationary “backer” bars, so as to form a closed loop path. The photoreceptor belt is driven about the closed loop path to present particular areas of the photoreceptor belt sequentially into association with electrographic process stations to form desired reproductions.
Debris particles on the drive roll can cause the coefficient of friction of the drive roller to drop appreciably. The buildup of debris particles on the backside of the photoreceptor belt can also adversely affect the drive roll friction and the drive performance of the photoreceptor belt as it is driven about the closed loop path and, ultimately, affect the overall performance of the reproduction apparatus.
Several mechanisms have been employed for cleaning the backside of the photoreceptor belt. One mechanism includes a stationary pad comprising a material such as cotton. This type of pad can easily become saturated with debris, with the period of time required for the pad to become saturated not readily predictable. Saturation of the pad can cause excessive abrasion and scratching of the photoreceptor belt, thereby necessitating frequent inspection and cleaning.
Printing apparatuses that run heavy (stiff) paper are more sensitive to the long standing problem of transfer white streaks. The aforementioned streaks can typically be 1-3 mm wide and are not present in the image on the photoreceptor before transfer. The streaks result from debris generated from the photoreceptor inner surface that can stick to the stationary transfer photoreceptor backer bar, thereby causing a high spot in the belt photoreceptor in the transfer zone This “tented” high spot causes higher local contact pressure between the paper-toner-photoreceptor interfaces in the transfer zone. This higher pressure causes a local degradation in transfer efficiency, with less toner making it to the paper. The residual toner (i.e., that should have been on the paper) remains on the photoreceptor and is subsequently cleaned off by a cleaner sub-system.
Light weight papers are generally not affected by the aforementioned increase in contact pressure. Light weight papers are able to conform to the slight waviness of the photoreceptor in the transfer zone. In contrast, heavy weight papers are unable to conform to the photoreceptor waviness, with resulting pressure non-uniformity and subsequent white streaks. Local photoreceptor waviness on the order of 30 microns has been shown to cause transfer streaks.